There are techniques of detecting drivers' dangerous actions and the positions thereof and emitting a warning when vehicles approach the detected positions. For example, a technique described in Patent Document 1 is a warning system mounted on a vehicle. This warning system detects dangerous driving of the vehicle that exceeds safety criteria, records the positional information of the vehicle when there is dangerous driving on the basis of the detected signal, and warns a user that a position showing the previously recorded positional information of the vehicle when there is dangerous driving is close to a current position.